Interpretive Summary: In our studies on preventing contamination of plant food products by the carcinogenic fungal toxin, aflatoxin, we found that growth of the responsible fungus, Aspergillus flavus, was inhibited by cecropin A. Cecropins, originally found in insects, are antimicrobial polypeptides deadly to bacteria and fungi. Studies presented here show for the first time that the lethality of cecropin A was inhibited by a membrane component of bacteria, lipopolysaccharide. The mechanism of the inhibition was of concern, particularly since bacteria are profuse inhabitants of food crops infected by A. flavus. We found that lipopolysaccharide molecularly binds cecropin A and thereby inactivates it. Thus, certain bacteria but not fungi have protection against cecropin's lethality.

Technical Abstract:
The ability of an antimicrobial insect polypeptide, cecropin A (CA), to control toxigenic microbes of plants was examined. CA was lethal to the aflatoxigenic fungus Aspergillus flavus and a bacterium implicated in byssinosis, Pantoea agglomerans. Lipopolysaccharide (LPS) inhibited the antibacterial activity. The role of molecular binding in the inhibition was assessed by competitive dyeing, circular dichroism, and solubility changes. CA prevented dyeing of LPS by dimethyl-methylene blue. LPS induced a conformational change in CA from no helical modes to 45% helix. An LPS subunit, Lipid A, formed an insoluble complex with CA. These results are consistent with molecular binding between LPS and CA. The binding inactivated CA; whether LPS became endotoxigenically inactive is of future interest.